The present invention relates to a fire-resistant glazing of the G fire-resistance classes comprising a pane made of silicate glass, which pane is toughened by the thermal route with air by means of a conventional toughening plant, and possessing safety glass properties.
Fire-resistant glazings of the G fire-resistance classes and their frames and their mountings must withstand, during a fire-resistance test in accordance with DIN Standard 4102 or with ISO/DIS Standard 834-1, exposure to fire and to smoke for a certain period of time. During this time, the panes may neither shatter under the effect of the stresses which appear as a result of the temperature gradients between the surface of the pane and the embedded edge nor exceed their softening point, because they would otherwise lose their stability and would thus free the opening. According to the tire in minutes during which they withstand fire, they are ranked in the G 30, G 60, G 90 or G 120 fire-resistance classes.
In general, fire-resistant panes are held in frames which protect, to a more or less large extent, the edges of the panes against the effect of heat. The temperature gradient which thus appears between the middle of the pane and the edge brings about considerable tensile stresses in the marginal region and results in destruction of the panes, if specific measures are not taken to compensate for these tensile stresses. These measures consist of a thermal toughening of the panes making it possible to induce strong initial compressive stresses in the marginal region. The thermal toughening makes it possible to confer additional safety glass properties on the pane when the toughening is carried out, so that, in the event of shattering, the pane fragments into tiny pieces,
It is in principle possible to measure the value of the initial stresses at the surface of the pane and in the marginal region by photoelasticimetry. This measurement by photoelasticimetry is, however, relatively expensive. In practice, success is consequently achieved in determining the state of initial stresses by means of the flexural/tensile strength obtained by the toughening, in accordance with DIN Standard 52303 or with EN Standard 12150. Experiments have, in this case, shown the necessity to provide a flexural/tensile strength of at least 120N/mm.sup.2 when the pane has to withstand tensile stresses generated by temperature gradients at the edge. In view of the fact that non-toughened panes exhibit a base flexural/tensile strength of approximately 50N/mm.sup.2, this means that it is necessary to increase this strength, via the toughening, by at least 70N/mm.sup.2. The value of this increase in the flexural/tensile strength corresponds directly to the value of the initial surface compressive stresses.
In addition, it is possible to increase the fire-resistance time by increasing the depth of insertion of the pane into the frame. In the case of a flexural/tensile strength of the pane of 120N/mm.sup.2 and of a depth of insertion of 10 mm, the glazing is, for example, in accordance with the G 30 fire-resistance class, whereas a depth of insertion of 20 mm makes it possible to achieve the G 90 fire-resistance class,
Panes made of ordinary float glass (silica glass based on soda and lime) can be appropriately toughened by means of conventional toughening plants, in view of the fact that these glass compositions exhibit relatively high thermal expansion coefficients of greater than 8.5.times.10.sup.-6 K.sup.-1. Ordinary float glass makes it possible to achieve flexural/tensile strengths which can range up to 200N/mm.sup.2. Under the effect of the tensile stresses brought about by the temperature gradients, the panes do not consequently shatter if the depth of insertion is approximately 10 mm but they lose their stability because of their relatively low softening temperature of approximately 730.degree. C. Toughened panes made of float glass are thus in accordance, under normal installation conditions, with at the very most the G 30 fire-resistance class.
However, monolithic panes of the G 60 fire-resistance class and of higher classes are also known. These panes are composed of compositions having a high softening temperature of greater than 815.degree. C. and exhibit, for this reason, a lengthy resistance time during a fire-resistance test. In this case, heat-resistant glasses based on borosilicate and on aluminosilicate prove to be particularly appropriate. However, these panes also have to be toughened by the thermal route in order to be able to withstand the high tensile stresses which appear in the marginal region during a fire-resistance test.
The use of thermal toughening for fire-barrier glazings made of heat-resistant glasses based on borosilicate or on aluminosilicate is known from the documents DE 23 13 442 B2 and DE 24 13 552 B2. According to these documents, only glasses for which the product of the thermal expansion a and of the modulus of elasticity E reaches 1 to 5 kp.cm.sup.-2..degree. C..sup.-1, that is to say glasses based on borosilicate or on aluminosilicate with a thermal expansion of .alpha..sub.20-300 =3 to 6.5.times.10.sup.-6 .degree. C..sup.1, are suitable for toughening. However, the toughening required at the edge of these panes cannot be achieved by means of ordinary air toughening plants but involves a specific process in which the panes are placed, during heating, between slightly smaller ceramic tiles, so that the edge of the pane sticks out from the ceramic tiles and is thus cooled more quickly, whereas the middle of the pane cools more slowly under the effect of the ceramic tiles. The toughening required at the edge can certainly be obtained in this way but the panes thus manufactured do not exhibit any safety glass properties.
It is known, from the document DE 43 25 656 C2, to use, for the manufacture of monolithic fire-barrier glazings, glasses which have a thermal expansion coefficient a of between 3 and 6.times.10.sup.-6 K.sup.-1, a specific thermal stress .phi. of between 0.3 and 0.5N/(mm.sup.2.K), a softening point (=temperature for a viscosity of 10.sup.7.6 dPa.multidot.s) of greater than 830.degree. C. and a transformation point (=temperature for a viscosity of 10.sup.4 dPa.multidot.s) of between 1190.degree. and 1260.degree. C. The specific thermal stress is the quantity specific to the glass calculated from the thermal expansion coefficient .alpha., from the modulus of elasticity E and from the Poisson coefficient m according to the formula .phi.=.alpha..E/(1-m). Panes exhibiting these physical properties can acquire, in a conventional air toughening plant, both the initial compressive stresses necessary at the edge and the toughening stresses exerted over the entire surface, which are necessary in order to obtain fragmentation in tiny pieces, so that no specific measure is necessary for the toughening and that the manufacturing process is thus greatly simplified thereby. Panes exhibiting these physical properties necessarily contain, however, B.sub.2 O.sub.3, Al.sub.2 O.sub.3 and ZrO.sub.2 in amounts which complicate the melting process and the transformation process. They cannot be manufactured according to the float-glass process, which has proved its exceptional profitability, given that their transformation point is too high and that the melting additionally requires specific measures.
Borosilicate-based glass compositions are known, from the document DE 28 18 804 B2, which are certainly designed for use in fire-barrier glazings and which, because of their relatively low transformation point, can melt according to the float-glass process and can also be toughened by means of ordinary toughening plants. These glasses contain, however, 11.5 to 14.5% B.sub.2 O.sub.3 and in addition exhibit physical properties similar to those of the glasses known from the document DE 43 25 656 C2. Even in the case of these glasses, the initial compressive stresses and the flexural/tensile strength which can be achieved with air toughening are limited to relatively low values and these glasses moreover exhibit the difficulties and disadvantages known during the melting of borosilicate-based glasses.